Why some patients have convulsion after fainting during blood taking?

A subset of people who faint will have some muscle jerking movement after the event. It is brief, subsides quickly and does not have the pattern of post event symptoms that seizures do. It is important to separate this from a true seizure, which could trigger a faint, and has long term implications. Proper evaluation by a doc is necessary to sort these out.

Syncope is due to decreased blood flow to the brain and becoming prone or supine restores this, but in some cases, the individual remains upright or sitting which prolongs the problem resulting in transient jerking of the extremities. This activity is not a garden variety of epilepsy and is quelled once assuming a flat posture.